1. Technical Field
The present invention relates to ultrasonic imaging apparatus and methods for imaging internal organs of a sample body using ultrasound.
2. Background Art
Ultrasonic imaging apparatus, such as those used for medical diagnostical imaging, utilize ultrasonic pulse reflection to display a tomographic image of soft tissues of a living body or an image of blood flowing in a living body for observation on a monitor practically in real time. Ultrasonic imaging apparatus are considered highly safe, because they do not expose the subject to radiation as diagnostical imaging apparatus employing radiation. In addition, they are small and inexpensive. For these reasons, ultrasonic imaging apparatus are widely used in the field of medicine.
An ultrasonic imaging apparatus produces images in the following way. An ultrasonic wave is transmitted to a subject from an ultrasonic probe made up of an array of piezoelectric elements. The position where the ultrasonic waves are focused inside the subject is controlled by adjusting the timing at which the ultrasonic waves are transmitted by each of the elements in the array. A region to be imaged is entirely scanned by the ultrasonic waves in this manner to produce a tomographic image. If the real-time nature of imaging can be disregarded, it is only necessary to bring both transmission wave and reception wave into focus at each point in the imaged region. However, in reality, in order to realize both a good image quality and the real-time feature, the ultrasound is generally focused at each point only during reception and not necessarily during transmission.
Specifically, during transmission, the ultrasonic waves are focused only at representative points along the line of propagation of the sound. On the other hand, during reception, the positions inside the subject at which the signal was reflected are estimated based on the timing of reception. The reception signals of the individual elements in the array are summed while introducing a predetermined delay such that the ultrasonic wave is focused at the position from where the reception signal was reflected. By varying the delay time, the reception focal point is varied. This system wherein the ultrasonic wave is received while the delay time is varied continuously is called the dynamic focus system. Once the information on one scan line is obtained, an adjacent scan line is scanned while the dynamic focusing operation is performed, and this is repeated to produce a two-dimensional tomographic image.
A description of the waveform of an ultrasonic wave transmitted to a living body by an ultrasonic imaging apparatus will follow. As for the ultrasonic waveform transmitted to the living body from a probe of the ultrasonic imaging apparatus, since the length of the waveform determines the distance resolution, a pulse wave with as short a length as possible along the time axis should preferably be used. On the other hand, the signal intensity should be large if the SN (signal to noise) ratio, which is the intensity ratio of signal to noise, is to be improved. However, the maximum value of the ultrasonic wave intensity should be limited by taking its influence on the living body into account. In order to increase the transmission energy under such a limitation, a coded transmission/reception method commonly used in radar applications is employed. In this method, a coded signal elongated along the time axis is transmitted. A reflection signal from the inside of the subject is received and converted into an electric signal. The electric signal is compressed along the time axis by filtering, so that the signal is transformed back to a pulse wave.
The coded transmission/reception method will be described by referring to FIG. 1. FIG. 1A shows the case where the transmission waveform is not coded, while FIG. 1B shows the case where the waveform is coded.
As shown in FIG. 1A, when the transmission waveform is not coded, as an ultrasonic probe 11 is energized by a drive pulse a1, an ultrasonic pulse with a form b1 is generated by the transfer function of the ultrasonic probe 11 in a process of transforming the electric pulse into an ultrasonic signal. The ultrasonic pulse is transmitted into the living body and reflected by a reflector 15 inside the body. The reflected pulse is received by the ultrasonic probe 11 and converted into an electric signal. The electric signal is converted into a digital signal by an A/D converter 12. Here the reception signal is again transformed by a transfer function, so that it comes to have a waveform c1 whose amplitude is the signal intensity. In ultrasonic tomographic imaging, the distance to the reflector 15 is determined by the time it takes for the pulse to return. Accordingly, the distance resolution is about equal to the width of the pulse waveform c1, which in the illustrated example is about three times the wavelength.
On the other hand, in the case of the coded transmission/reception method, a drive coded pulse a2 elongated along the time axis is used, as shown in FIG. 1B. As the ultrasonic probe 11 is driven with this drive coded pulse a2, an ultrasonic signal with a waveform b2 is transmitted into the living body from the ultrasonic probe 11 and reflected by the reflector 15. The reflected signal is again converted into an electric signal by the ultrasonic probe 11, thereby producing a receive waveform c2. By using a decoding filter 13 corresponding to the drive coded pulse a2, the waveform c2 is compressed along the time axis by the amount by which the drive signal has been elongated. As a result, a demodulated waveform d2 is obtained which has a similar level of distance resolution and a larger signal intensity compared with the receive waveform c1. Thus, transmission energy can be increased without increasing the amplitude inside the living body. Namely, according to the coded transmission/reception method, the reflected ultrasonic wave, after passing through the subject, is converted into an electric signal by the ultrasonic probe and compressed along the time axis while maintaining its signal energy, so that the reception signal is converted into a pulse signal having a large peak value.
This technique is combined e.g. with the known Barker code or Golay code, and a mismatched filter for minimizing the side lobe as described in Robinson et al., “Geophysical Signal Analysis,” Prentice-Hall (1980), pp. 191-212. Specifically, the use of a code such as the Barker code having small sidelobes in its auto-correlation function, as the transmit code, enables the receive signal to be easily converted back to a pulse signal. A decoding filter producing auto-correlation function as the output can be an excellent filter because of its excellent capability to extract a signal out of noise. However, when a Barker coded signal is decoded by itself, there are still, though small, sidelobes of a minimum of about −20 dB, which is within the dynamic range of the image. Thus, a decoding filter producing the auto-correlation function is not good enough as a decoding filter in this case. The mismatched filter is a modification of the decoding filter in which the sum of squared errors with respect to a simple delta function having no sidelobes is minimized. Examples of the application of this method to ultrasonic imaging apparatuses are disclosed in IEEE Transaction on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 45, No. 2, pp. 349-355 (1998), and Vol. 45, No. 1, pp. 98-113 (1998); and JP Patent Publication (Nonexamined Application) Nos. 11-309145, 11-309146, and 11-309147, for example.
It is difficult to combine the above-described dynamic focusing technique and coded transmission/reception technique, as illustrated in FIG. 2. For simplicity's sake, two reflectors, 21a and 21b, are considered. Echo signals from each of the reflectors 21a, 21b are each distributed within a circle with the position of each reflector at the center of each circle, as shown. Reception timings at the ultrasonic probe vary by an amount corresponding to the difference in the positions of the reflectors. If the coded transmit wave is longer than the distance between the reflectors 21a and 21b, the echo signals are superposed on the other in reception. If the individual elements on the ultrasonic probe share the same degree of superposition, the coded signal can be restored by passing the superposed signals through a decoding filter after beamforming.
In the case of dynamic focusing, however, because delay time 22a and 22b after the reception phase alignment and summing vary continuously depending on the reception timing, that is the estimated position of the reflector. As a result, the degree of superposition of the echo signals from the two points varies depending on the position of each element on the ultrasonic probe. If the phase alignment and summing process is conducted for all the elements uniformly and the elements are bundled, the effect of coding will not be uniform between the individual apertures and, as a result, it becomes impossible to decode the signals. Here, the aperture is the span of the portion of the all elements making up the ultrasonic probe which is actually used. More specifically, a transmit aperture is the area in which those elements of the ultrasonic probe that are used for transmitting ultrasonic waves are arranged. A receive aperture is the area in which elements for receiving ultrasonic waves for subsequent processes are arranged. Although there is a method of decoding the codes before the phase alignment and summing process, this method requires a large-sized circuitry, which means losing one merit of the ultrasonic imaging apparatus—its inexpensiveness. Particularly, in the case of three-dimensional imaging using a two-dimensional array probe, the number of elements ranges from one to several thousands. Thus, it is virtually impossible to attach a decoder to each element.
It is, therefore, an object of the invention to combine the coded transmission/reception and dynamic focusing techniques without requiring large-sized circuitry or giving rise to unwanted responses that are typically associated with the coded transmission/reception technology.